Is it possible to use compositing to remove patterns in rendered images?
You could use the Mist pass with a color ramp and color invert to strengthen it, or you could do like the included image, enable an Object Index pass so you could then get a strong mask and invert it to affect the areas outside of the object. In this way, even if you need to render frames again, you can lower your settings all the way so that the result is as small as possible and just use the pass to affect the frames you already have.